The present invention relates to a combustion furnace or reactor with a multi-stage fluidized bed system.
There have been recently devised and demonstrated boiler furnaces employing a fluidized bed combustion system for burning coal, but further extensive research and development is needed to reduce the emission of nitrogen oxides NO.sub.x and sulfur oxides SO.sub.x to a minimum. To this end, there has been devised and demonstrated a system wherein particles of lime stone are used as bed materials or particles so that NO.sub.x and SO.sub.x may be removed by calcium compounds which can absorb sulfer oxides and can act as catalysts for reducing NO.sub.x. The boiler furnaces with a single fluidized bed of particles of lime stone have been used in practice, but they are not satisfactory in view of the higher pollution control standards.
The main reason is that even though the removal of nitrogen oxides NO.sub.x may be made most effectively in the reducing atmosphere while the removal of sulfur oxides SO.sub.x, in the oxidizing atmosphere, only one fluidized bed is used to effect both desulfurization and denitrification.
In order to overcome the above problem, according to the present invention, the furnace or reactor is divided into a lower denitrification zone and an upper desulfurization zone wherein the fluidized beds of particles of lime stone are formed. Since the lower and upper zones may be optimumly operated independently of each other, effective denitrification and desulfurization may be attained to an extent hitherto unattainable by any prior art single fluidized bed systems.
In order to attain satisfactory denitrification and desulfurization and to inhibit the production of thermal NO.sub.x with the combustion furnace or reactor with a multi-stage fluidized bed system of the type described above, the following factors must be taken into consideration:
(1) Combustion air must be charged in such a way that the lower fluidized bed may have the reducing atmosphere while the upper fluidized bed may have the oxidizing atmosphere.
(2) Calcium compounds such as CaO, CaSO.sub.3, CaSO.sub.4 and the like which act as catalysts in denitrification must be positively supplied from the upper desulfurization zone to the lower denitrification zone.
(3) All of the reducing gases resulting from the lower denitrification zone must be made to pass through the upper desulfurization zone and burned with the secondary combustion air so as to burn unburned combustables.
(4) Production of thermal NO.sub.x must be suppressed as much as possible in the upper desulfurization zone.
The primary object of the present invention is therefore to provide a combustion furnace or reactor with a multi-stage fluidized bed system which may satisfactorily meet all the conditions described above. The present invention will become more apparent from the following description of preferred embodiments thereof in conjunction with the accompanying drawings.